This invention relates to medical implants with improved wear resistance, formed of a polymeric material such as ultra-high molecular weight polyethylene, a method for making the implants, and a container used to process the implants during thermal and radiation treatment.
Various polymeric materials have been used for the preparation of orthopaedic total joint replacement devices. Among them, olefinic polymeric materials such as ultra-high molecular weight polyethylene (UHMWPE) are especially useful polymeric bearing materials because of their physical properties and in vivo compatibility. UHMWPE has been defined as a predominantly linear type polyethylene which has a relative viscosity of 2.3 or greater at a solution concentration of 0.05% at 135.degree. C. in decahydronaphthalene. The nominal weight average molecular weight between 400,000 to 10,000,000; more usually from three to six million.
Orthopaedic implants may be manufactured by machining UHMWPE raw material, supplied in the form of polymer rods and slabs, into the desired shape of the orthopaedic implant devices. Alternatively, the implant devices can be produced directly by compression molding of the UHMWPE polymer powder. After manufacture, the components go through a sterilization procedure, usually after being placed inside a sealed package made of impermeable materials.
Several sterilization methods may be used, such as ethylene oxide, heat, or radiation. Radiation treatment is preferred over ethylene oxide and heat treatment because ethylene oxide has toxicity concerns, and heating the manufactured implant beyond its annealing point can ruin it by altering its precise physical dimensions. Furthermore, heat can also destroy the integrity of the sealed package containing the implant. Suitable radiation methods include the use of x-ray, electron beam and gamma radiation to irradiate the implant according to standard sterilization criteria.
Unfortunately, high energy radiation of the type required for sterilization may destabilise the polymeric material, such as UHMWPE because of the generation of free radicals effected by radiation, especially in the presence of oxygen. If the implant is instead irradiated in an oxygen reduced, or oxygen free atmosphere, the radiation induced free radicals are reduced in concentration due to their reaction with neighboring free radicals to form carbon-carbon cross-links.
The prior art has recognized that the less the free radical concentration, the better the polymer material retains its physical properties over time. Conversely, the greater the free radical concentration, the greater the potential for physical properties of the implant to degrade over time. Several prior art patents and articles have reported methods which enhance UHMWPE physical properties by reducing free radical concentration. U.S. Pat. No. 5,037,928 titled xe2x80x9cProcess of Manufacturing Ultrahigh Molecular Weight Linear Polyethylene Shaped Articlesxe2x80x9d issued on Aug. 6, 1991 to S. Li, et al. discloses a prescribed heating and cooling process for preparing a UHMWPE exhibiting a combination of properties including a creep resistance of less than 1% without sacrificing tensile and flexural properties produced by a high compression process. U.S. Pat. No. 4,813,210 titled xe2x80x9cRadiation-Sterilized Packaged Medical Devicexe2x80x9d issued on Mar. 21, 1989 to T. Masuda, et al. discloses a packaging method where a medical device which is sealed in a sterile bag, after radiation/sterilization, is hermetically sealed in an oxygen-impermeable material together with a deoxidizing agent for prevention of post-irradiation oxidation and additional free radical formation U.S. Pat. No. 5,160,464 titled xe2x80x9cPolymer Irradiationxe2x80x9d issued on Nov. 3, 1992 to I. M. Ward, et al. discloses a heating and irradiation process of oriented polyethylene having a weight average molecular weight less than or equal to 350,000 which produces improved strain rate sensitivity.
U.S. Pat. No. 5,414,049 titled xe2x80x9cNon-oxidizing Polymeric Medical Implantxe2x80x9d issued on May 9, 1995 to D. C. Sun, et al. discloses a method for heating, melting, and annealing UHMWPE raw material stock in an oxygen reduced atmosphere, to remove residual oxygen and moisture prior to forming the implant device. The forming process is also done under reduced oxygen atmosphere and the finished implant is later packaged in a sealed, air tight package. Afterwards, the packaged implant is irradiated with gamma radiation at ambient temperature, and then heat treated for several days in the absence of irradiation to cause free radicals to form self cross links without oxidation. At no time during the process does the temperature exceed the distortion temperature of the implants. For UHMWPE, the applicable processing temperature range is between about 25.degree. C. and about 140.degree. C.
Although the above referenced prior art methods may succeed in stabilizing the medical implant by reducing the free radical concentration, the additional cross linking caused by the irradiation and heating processes employed often cause a substantial change in implant physical properties, most notably a decrease in ductility (see e.g. D. C. Sun et al., Material Property Comparison of Surgical Implant Grade UHMWPE and xe2x80x9cEnhancedxe2x80x9d UHMWPE; 20th Annual Meeting of the Society for Biomaterials, Apr. 5-9, 1994, Boston, Mass.). The consequences of changes in physical properties, aside from the desirable increase in wear resistance, is largely unknown with respect to the function of the implant in vivo, and therefore should be avoided. One reason given is that a decrease in ductility and an increase in stiffness may concentrate stresses in a smaller volume of the implant, and perhaps lead to premature mechanical failure of the polyethylene implant.
The present invention relates to a prefabricated polymeric implant displaying a large increase in wear resistance while simultaneously showing minimal changes in ductility and other mechanical properties, and the process for producing the improved implant. The present invention also relates to containers used to process the packaged implants.
The improved implant, having previously been sealed in an impermeable package under a reduced oxygen atmosphere, is produced by the process comprising the steps of preheating the implant, without irradiation, to a predetermined temperature, followed by irradiation of the preheated implant with a predetermined quantity of electromagnetic radiation while the implant is maintained within a predetermined temperature range, and finally controlled cooling of the irradiated and heated implant at a predetermined rate; with the overall requirement that the temperature of the packaged, polymeric implant is never allowed to reach its annealing or thermal distortion temperature at any time during the process.
Thermal conductive and thermal insulation containers are also provided for holding the packaged implants during the above heating, irradiation, and cooling process steps. The thermal insulated container is adapted to hold the thermal conductive container and the thermal conductive container is adapted to hold at least one prefabricated implant in a sealed package.
For the purpose of illustration, UHMWPE will be used as an example to describe the invention. However, the inventive process should also apply to other polymeric materials such as polypropylene, high density polyethylene, polyester, nylon, polyurethane, poly(methylmethacrylate, and other polymeric materials capable of free radical formation when irradiated by gamma radiation in the 1.0 to 100.0 MRads range.
The thermal containers are so configured to facilitate the processing of the packaged implants, by obtaining the optimum heating, irradiation, and cooling for the packaged implants contained therein.
Preferred embodiments of the invention will be described below in the context of the medical implant made by the inventive process, the method of making the improved medical implant, and the thermal conductive and thermal insulation containers which are useful in the inventive process.